The present invention refers to a twisting method and fixture for twisting free end portions of bar conductors, in particular for stator or rotor windings of electric machines.
It is known to provide stators or rotors of electric machines, like electrical generators or motors, for example for use in hybrid electrical vehicles (HEVs), wherein the stator or rotor winding is composed of a plurality of bar conductors, which are bent and variously interconnected to each other in order to provide so called bar windings.
In particular, the known art comprises bar windings composed of bar conductors having a rectangular cross section, where the term “rectangular” refers to a square section as well as to a “flat” section, generally indicating a rectangular section, wherein two sides of the section are smaller than the other two.
So called bar conductors are usually preformed by “U” or “P” bending, starting from linear bar conductors. U.S. Pat. No. 7,480,987 describes an exemplary method for pre-forming bar conductors (called “hairpin conductors” in this document). “U” or “P” preformed conductors, often also called “basic preformed conductors” in the technical field, typically have two adjacent legs, of different length, each provided with a free end portion and an opposed end portion, which is joined, by means of a joining portion, to the other leg.
For example, in order to provide a stator, it is known to carry out two different types of twisting on the “U” or “P” preformed conductors.
In a first type of twisting, also called “twisting from the insertion side”, the basic preformed conductors are adequately inserted in corresponding radially aligned pockets, which are provided in a twisting device, suitable for deforming, after insertion, such conductors. The twisting device is essentially used for “driving apart” the legs of the “U” or “P” shape, in order that both legs of same conductor, after extracting the latter from the twisting device, may be successively inserted into slots of a stator core, which are radially offset to each other by a predetermined pitch.
Published patent application US 2009/0178270 describes an exemplary “insertion side” twisting method, for uniform-pitch twisting of preformed bar conductors after their insertion into pockets of a twisting device.
After having undergone the first type of twisting, the bar conductors are inserted into the slots of the stator core through a first side of same (the so called “insertion side”) with respective free end portions protruding from a second side of core (the so called “welding side” or “connection side”), which is opposed to the first side.
The free end portions protruding from the welding side are then subjected to a second type of twisting, also called “welding side twisting”, after being inserted into pockets provided in a suitable twisting fixture. The twisting fixture's goal is to bend (“twist”) the free end portions of conductors in order to adequately shape such end portions and therefore allow the provision of adequate electrical connections between conductors in order to complete the winding.
It is to be noted that precision bending of free end portions of conductors facilitates the forming of connections among conductors. However, for various reasons, it may be difficult to correctly and precisely bend the free end portions of conductors, as required. For example, since the protrusion of the majority of these end portions on the welding side is relatively small, it may be difficult to access the end portions of conductors and carry out the operations required to ensure a correct bending both in the circumferential and in the axial direction with respect to the stator core axis. Moreover, as an example, a precise shaping of conductors is complicated by the fact that these are provided with an intrinsic elasticity, which, after bending, tries to drive them partially back to their starting configuration.
Published US patent application 2009/0302705 describes an exemplary welding side twisting method of above said type. The method described in this patent application allows at one time to achieve non uniform twisting of free end portions of bar conductors. In order to provide such twisting, the patent application describes a twisting fixture comprised of a pocket member, which is provided with a lost motion member, for defining a pocket of pocket member. In particular, the lost motion member is mounted circumferentially movable with respect to a main structure of pocket member.
It is felt the necessity to provide an alternate twisting method, with respect to the above said known art, for twisting free end portions of bar conductors for bar windings of electric machines.
The present description's general objective is to provide a twisting method, which is able to satisfy above said necessity.
This and other objectives are reached by a twisting method as defined in claim 1 in its more general form, and in its dependent claims, in some specific embodiments of the same.
A further aim of the present invention is to provide a twisting fixture as defined in claim 6 in its more general form, and in the dependent claims, in some specific embodiments of the same.